1. Field of the Invention
The present invention relates a method for producing high-strength steel, particularly a steel material for a welded structure, e.g., for a pressure vessel, bridge, or construction machine, in which both high strength and weldability are required. More particularly, the present invention relates to an inexpensive method for producing a high-strength steel having a tensile strength of 80 kgf/mm.sup.2 or more, even 90 kgf/mm.sup.2 or more.
2. Description of the Related Art
Heretofore, high-strength steels for welded structure use have been produced by heat-treating alloyed steels, i.e., by off-line quenching an tempering of alloyed steels. A large amount of various alloying elements are necessary for obtaining the high strength. This not only increases the production cost, but also necessitates a high preheating temperature prior to welding so as to prevent weld cracks.
Japanese Examined Patent Publication (Kokoku) No. 41-2763 discloses a precipitation-hardening method for strengthening steel. In this method, steel with copresent molydenum (Mo) and niobium (Nb) is ordinarily quenched and tempered off-line, so that the steel is strengthened by Mo-Nb precipitates. In addition, since the quenching temperature is approximately 900.degree. C. and is too low to solid-dissolve Nb and Mo greatly into a matrix, addition to the steel of a large amount of Nb and Mo is required so as to attain satisfactory precipitation hardening. This results in not only increased costs, but also problems in weldability, especially weld cracks. In order to avoid weld cracks, the steel must be preheated at a high temperature. There is thus a strong demand among users for steel for not requiring high-temperature preheating.
It is well recognized in the art that, in order to produce high-strength steel having a tensile strength of 80 kgf/mm.sup.2 or more and improved weldability, a few percent of nickel (Ni) and occasionally Mo, may be added to the steel and the steel quenched and tempered. One of the prior proposals is found in Seitetsu Kenkyu Vol. 273 (1971) pp 9904 to 9921. However, such prior proposals are disadvantageous in that the production cost is high and the weldability is impaired due to the high amount of alloying elements.
It is known from Tetsu to Hagane Vol. 67 (1981)'81-S1330 that the mechanical properties of an Nb-containing steel are enhanced by means of on-line accelerated cooling applied for producing line pipe materials. In this method, however, controlled rolling must be carried out prior or the accelerated cooling, and the stop temperature of accelerated cooling must be raised.
In addition, Tetsu to Hagane Vol. 68 (1982) '82-S1443 describes a direct quenching method followed by tempering, for producing high tensile steel. No consideration is found in this publication as to producing steel having a high tensile strength and improved weldability under the as-quenched state.